1. Technical Field
The present invention relates to a data hiding method for hiding message data into media data and a data extracting method for extracting hidden data.
2. Prior Art
With the development of multimedia society, large quantities of digital video and audio information have been circulated on internet systems or as CD-ROM software. For digital video and audio information, any individual can easily create a perfect copy without degradation, so the illegal use and copyright protection are becoming problematic. In order to prevent a third party from illegally copying media data such as video and audio data, the technique of hiding additional information, such as the signature of an author, into original media data is becoming the focus of attention. When digital video data or other similar data is illegally copied, it could be known whether or not the copy is an illegal one by confirming the signature hidden in the copy and specifying the source. A hiding technique such as this is called data hiding.
As for the data hiding technique, a technique such as mentioned as follows is disclosed in "Nikkei Electronics 4-22 1996," issued on Apr. 22, 1996. FIG. 1 is a block diagram for explaining a conventional data hiding and extracting method. In the process of hiding an identification (ID) information, first, the frequency transform of media information, such as original dynamic images, still images, photographs, or audio, is performed by discrete cosine transform (DCT) or high-speed discrete Fourier transform (DFT) to obtain its frequency spectrum. Spectrum diffused and convoluted to this frequency spectrum of the media information is ID information. The ID information is a random specified to each purchaser of a work. The random-number generating algorithm employs a normal distribution, and the length is 1000. Then, the frequency spectrum of the media information to which the ID information was added is restored by inverse frequency transform to the original digital work having the hidden ID information. The digital work has the hidden ID information but is almost identical with the original work, so the purchaser cannot perceive the difference visually.
Conversely, in the process of extracting the ID information, first, the frequency transform of the digital work, thought of as an illegal copy, is performed to obtain its frequency spectrum and the frequency spectrum of the original work is also obtained. Then, the difference between these spectra is taken and compared with the ID information issued by the author. With this comparison, the purchaser of the work can be specified, so it can be judged whether or not the copy is illegal.
One of the features of the aforementioned technique is that frequency space is utilized for hiding data. That is, media information is transformed from real space to frequency space, and the frequency component in the frequency spectrum is operated based on ID information. Another feature is that a frequency region to be utilized is local. That is, the high-frequency component in the frequency spectrum of media information is not utilized and spectrum diffusion is performed only for a low-frequency region. The reason for this is that if ID information is diffused up to a high-frequency region, ID information is erased in compressing or decompressing the image. That is, this conventional technique spectrum-diffuses ID information locally in the frequency space of media information and hides the ID information in the entire real space of the media information.
The aforementioned conventional technique, however, has the disadvantage that message information (ID information) can be easily filtered out from media information by employing a frequency filter. For example, when media information with message information hidden therein is processed by employing a high-pass filter which filters out only a low-frequency component, the component of the high-frequency band remains unfiltered, but the component of the low-frequency band in which message information has been hidden is completely filtered out. Therefore, it is no longer possible to extract ID information from the media information that was output from a high-pass filter, and it is possible to hide different additional ID information.